Local Orthorhombicity In The Magnetic C-4 Phase Of The Hole-Doped Iron-Arsenide Superconductor Sr1-Xnaxfe2as2

We report on temperature-dependent pair distribution function measurements of Sr1-xNaxFe2As2, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic C-4 phase. Quantitative refinements indicate that the instantaneous local structure in the C-4 phase comprises fluctuating orthorhombic regions with a length scale of similar to 2 nm, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic C-4 phase and the neighboring C-2 and superconducting phases.